//等差数列划分II - ⼦序列（hard）: https://leetcode.cn/problems/arithmetic-slices-ii-subsequence/description/
class Solution {
public:
    int numberOfArithmeticSlices(vector<int>& nums) {
        int n = nums.size();
        // 优化
        unordered_map<long long, vector<int>> hash;
        for (int i = 0; i < n; i++)
            hash[nums[i]].push_back(i);
        vector<vector<int>> dp(n, vector<int>(n)); // 创建 dp 表
        int sum = 0;
        for (int j = 2; j < n; j++) // 固定倒数第⼀个数
        {
            for (int i = 1; i < j; i++) // 枚举倒数第⼆个数
            {
                long long a = (long long)nums[i] * 2 - nums[j]; // 处理数据溢出
                if (hash.count(a))
                    for (auto k : hash[a])
                        if (k < i)
                            dp[i][j] += dp[k][i] + 1;
                        else
                            break;
                sum += dp[i][j];
            }
        }
        return sum;
    }
};
